Refrigerant compressors are widely used in cooling devices for the industrial, commercial, and private sectors. These devices operate with mechanical compressors which compress the refrigerant, liquify it in the condenser by cooling with air, water, or another medium, and evaporate it in the evaporator, thereby absorbing heat from the medium to be cooled. The refrigerant employed for large-scale plants is primarily ammonia; fluorochlorohydrocarbons, such as dichlorodifluoromethane or chlorotrifluoromethane, are also used for large scale plants, and principally for commercial refrigeration plants and domestic appliances.
Highly refined (generally naphthene-based) mineral oils similar to white oil are used for the lubrication of refrigerant compressors. Alkylaromatics and poly-alpha-olefins are employed as synthetic oils for this purpose.
The function of the lubricating oils is to lubricate the moving components of the compressor, remove heat from the hot components of the compressor, and seal off the compression space and the valves. These requirements also determine the properties which the lubricating oils must satisfy. They must withstand thermal stresses and also remain capable of flowing at evaporator temperatures. Moreover, it should be remembered that the lubricating oils may be discharged from the compression space into the refrigerant circulation and cannot be removed completely by subsequent oil separators. They must therefore be miscible with the refrigerant over wide ranges of temperature and concentration, so that recycling of lubricating oil which has entered the refrigerant circulation in the compressor is ensured.
Fluorochlorohydrocarbons have for some time been suspected of damaging the ozone layer of the Earth's atmosphere. Efforts are therefore being made to limit them to those uses where they cannot be replaced. Attempts are otherwise being undertaken to replace them by substances which have an equivalent action but are harmless. Chlorine-free, partly fluorinated hydrocarbons, such as 1,1,1,2-tetrafluoroethane, 1,1,1,2,3,3,3-heptafluoropropane, pentafluoroethane, 1,1,1,3,3,3-hexafluoropropane and trifluoromethane, will be employed in the future as refrigerants. These substances are distinguished by high heat stability and desirable thermodynamic properties in the temperature ranges in which refrigerant compressors operate.
The chlorine-free aliphatic fluorohydrocarbons are miscible with the lubricants used to date for refrigeration plants only to a slight extent. The two classes of substances form two-phase mixtures over wide concentration ranges, so that recycling of the lubricant discharged from the compressor is considerably impeded, especially at low temperatures. It has therefore been necessary to discover novel lubricants which are compatible with the chlorine-free refrigerants. Amongst these novel lubricants, esters obtained from monocarboxylic acids and di- or polyhydric alcohols have proven outstandingly suitable (cf. DE 40 06 827 Al). More recent studies are aimed at developing lubricants from readily available raw materials and, in view of the number of new refrigerants, with a large variety of fluorohydrocarbons.
Thus, the object of the Invention is to provide inexpensive starting materials which can be converted by the conventional route into lubricants which can be used with a wide variety of refrigerants.